🚀 go-pugleaf

"Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU> wrote:

>The issue is not merely semantic or pedantic, I think.

Not merely but mainly  a problem of terminology :-))

> When you said
>"Forces cause acceleration (not necessarily movement)" that seemed to me
>contradictory, for how can you have acceleration without movement? 

Of course you can: imagine two "identical" cars with identical
velocity heading towards each other (velocities aligned) an
crashing "totally" inelastic (I hope the term is correct). The
result would be: _first_ a rather big amount of acceleration for
both objects and _second_  a bunch of torn metal lying there and
no trace of "movement" left. (This sure is drastic..). 

I'm putting a bit of weight on this because the "chain": force ->
movement, or [no force]->[no movement] is obvious in our
everyday-life (thanks to friction) and so many students/pupils
are very hard to convince of another concept.

>Another problem I have with your terminology is this: You write:
>        Forces cause acceleration
>        Couples cause a torque

>But the two sentences, which seem to be constructed to emphasize
>parallelism of thought, or perhaps analogous concepts, are *not* strictly
>parallel in meaning. They *would* be if the second were worded:

Thats ok. They were not meant to express parallelity but to be a
correction to
>>Forces produce translation; couples produce rotation. (Mervin K.)

>> >A couple is defined to be the torque produced by two *equal* size and
>> >oppositely directed forces with a displacement between their lines of
>> >action.  [Statement A]
>....
>> >The torque due to a couple is obviously Fd where F is the size of
>> >either one of the forces and d is the perpendicular displacement between
>> >them.   [Statement B]
>>
>> You're making contradictions in your paragraph:
>> "A couple is defined to be the torque ..." vs. "The torque due to
>> a couple is...". Between these two point of view I prefer the
>> second: two forces with nonzero perpendicular displacement cause
>> a torque and this causes an angular acceleration.

>They aren't two points of view, Guntbert. I've numbered my two
>statements, which you consider 'contradictory'. Statement [A] is a
>definition, and a correct one at that, though use of the phrase 'parallel
>and non-colinear' would have made it clearer and more concise.

>Statement [B] probably bothers you because of the phrase 'Torgue *due to*
>a couple'. I shall challenge you on this one, to tell me *specifically*
>what fault you find with it.

I take that challenge: Your mistake is a logical one. If [A] is
true then there is no reason why one of two equivalent objects
(lets say torque) should be *due* to the other, as stated in [B].

>special behavior: "The torque of a couple is the same wherever it is
*Thats it: The torque of a couple...*

>applied to the body." This is not true of all torques. That is, the
>effect of a couple on a body is the same wherever it is applied.

>There is a semantic point that the world 'couple' has two uses. (1) It
>describes the particular combination of two equal and opposite
>non-colinear forces. (2) It describes the torque due to such a
>combination of forces.

Ok. In German (I'm Austrian) we use only (1)
>> habits": You can express one law as: "acceleration and force are
>> proportional". => Accelerating the CM leads by inertia to a force
>> in the opposite direction....

> Now I wonder whether you mean the
>'fictitious' force you'd put in to make F=ma when you do a problem in an
>*accelerating frame of reference*. (In the same class with coriolis,
>centrifugal, and such forces.) 
Yes, I confess, I did.
>In any case, this seems totally
>unnecessary in the present case: the problem of the force acting on the
>stick in space.

No, it is not. Or how will you convince anybody to see the
importance as to WHERE the force strikes.  (Or the importance of
CM). The only reason: there are only few "natural" axes of
rotation, and CM lies on all of them, because it WON't move if
you apply a force with a nonzero perpendicular displacement in
regard of CM.


>Your posting, and the others, stimulated us to think more deeply about
>this problem than we are used to doing. That's good for us all. It also
>stimulated us to be more attentive to the words and terms we use.

I would like to return that last paragraph to you. 
	tnx
	Guntbert
------------------------------------------------
Guntbert Reiter : guntbert@borg-6.borg-graz.ac.at
HTL Weiz
Graz
Austria